The present invention relates to a process for preparing propene by metathesis of olefins. Olefin metathesis (disproportionation) in its simplest form describes the reversible, metal-catalyzed rearrangement of olefins by cleavage and reformation of Cxe2x95x90C double bonds. For example, olefins of the formulae R1xe2x80x94CHxe2x95x90CHxe2x80x94R2 and R3xe2x80x94CHxe2x95x90CHxe2x80x94R4 are reversibly reacted to form olefins of the formulae R1xe2x80x94CHxe2x95x90CHxe2x80x94R3 and R2xe2x80x94CHxe2x95x90CHxe2x80x94R4. In the metathesis of acyclic olefins, a distinction is made between self-metathesis in which an olefin is converted into a mixture of two olefins having different molar masses and cross- or co-metathesis in which two different olefins react. An example of self-metathesis is the reaction of two molecules of propene to give ethene and 2-butene, as is performed, for example, by the Phillips triolefin process, see Hydrocarbon Processing, Volume 46, November 1967, No. 11, p. 232. An example of cross-metathesis is the reaction of propene and 1-butene to give ethene and 2-pentene. If one of the reactants is ethene, the reaction is customarily referred to as an ethenolysis.
The metathesis reactions are carried out in the presence of catalysts. Suitable catalysts for this purpose are, in principle, homogeneous and heterogeneous transition metal compounds, in particular those of transition groups VI to VIII of the Periodic Table of the Elements, as well as homogeneous and heterogeneous catalyst systems in which these compounds are present.
DE-A-19 40433 discloses the metathesis of 1-butene with 2-butene to form propene and 2-pentene, with Re2O7/Al2O3 being used as catalyst. The 2-pentene formed is reacted further with sodium hydride on potassium carbonate and ethene to give heptenes.
The metathesis of 1-butene and 2-butene to give propene and 2-pentene is mentioned in K. L. Anderson, T. D. Brown, Hydrocarbon Processing, Volume 55, August 1976, No. 8, pp. 119-122 as a secondary reaction in the synthesis of isoamylene.
EP-A-0 304 515 discloses a metathesis process for reacting 1-butene with 2-butene to give propene and pentenes, which is carried out in a reactive distillation apparatus using Re2O7/Al2O3 as catalyst.
U.S. Pat. No. 3,526,676 discloses the metathesis of 1-butene with 2-butene to give propene and pentene. It is carried out over MoO3 and CoO on AM203.
U.S. Pat. No. 3,785,957 discloses a process for the production of fuel having a high octane number. In this process, an olefinic fuel is disproportionated together with ethylene and the product is fractionated into a propene stream, a butene stream, a C5-or C5-C6-olefin stream and a C6+ or C7+ fuel stream. The C5-or C5-C6-olefin stream is disproportionated with ethene over a WO3/SiO2 fixed-bed catalyst to give propene and butenes. The propene obtained is disproportionated to form ethene and butenes, and the butenes are alkylated with isobutane.
U.S. Pat. No. 3,767,565 discloses a process for increasing the octane number of fuel in which a C5 fraction of an olefinic fuel is reacted with ethene in the presence of a catalyst comprising WO3/SiO2 and MgO to form ethene, propene, n-butenes and isobutenes. The propene obtained is disproportionated and the resulting n-butenes are alkylated with isobutane.
EP-A1-0 691 318 discloses an olefin metathesis process in which C5-olefins and ethene are reacted in the presence of a catalyst to give mixed C4-olefins and propene. Thus, 2-methyl-2-butene is reacted with ethene to give isobutene and propene. A mixture of 2-pentenes and 2-methyl-2-butene is reacted to give a mixture of 1-butene, isobutene and propene.
A process for preparing propene in high yield by reacting 1-butene, 2-butene and isobutene is not known.
A process for preparing propene in high yield without using an excess of ethene is not known. In the above processes, propene is prepared with addition of at least equimolar amounts of ethene. To achieve high propene selectivities, large amounts of ethene have to be circulated. In addition, isobutene present in the feed is not reacted and reduces the space-time yield.
It is an object of the present invention to provide a process for preparing propene in high yield from mixtures comprising 1-butene, 2-butene and isobutene. This should be able to be carried out without using an excess of ethene. It should be possible to obtain propene from C4 streams which are low in 1-butene using very little ethene, and isobutene should also be converted into the desired product.
We have found that this objects is achieved by a process for preparing propene by reacting 1-butene, 2-butene and isobutene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements, and subsequently separating off the propene.
We have also found that this object is achieved by a process for preparing propene by
a) reaction of 1-butene, 2-butene and isobutene to give propene, 2-pentene and 2-methyl-2-butene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements,
b) subsequent separation of the propene and 2-pentene/2-methyl-2-butene formed,
c) subsequent reaction of the 2-pentene and 2-methyl-2-butene with ethene to give propene, 1-butene and isobutene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition group VIb, VIIb or VIII of the Periodic Table of the Elements,
d) subsequent separation of the propene and 1-butene/isobutene formed,
e) subsequent return of the 1-butene and isobutene formed to step a).
The preferred process of the present invention comprises 2 metathesis steps. In the first step, 1-butene, 2-butene and isobutene are reacted to give propene, 2-pentene and 2-methyl-2-butene. In a second step, 2-pentene and 2-methyl-2-butene are then reacted with ethene to give 1-butene, isobutene and propene. According to one embodiment of the invention, 1-butene and isobutene are returned to the first reaction step.
The net reaction is thus the reaction of 2-butene with ethene to form 2 molecules of propene. According to one embodiment of the invention, the reaction of 2-pentene and 2-methyl-2-butene with ethene formally requires only equimolar amounts of starting materials in order to obtain the products in high yield. Thus, in contrast to the reverse trioolefin process, the amount of ethene used can be kept small.
Both metathesis steps can be carried out as a reactive distillation, as is described below.
According to one embodiment of the invention, 1-butene, 2-butene and isobutene can be used in the reaction as pure substances. According to another embodiment of the invention, the butenes are used in the form of a C4 stream which originates, for example, from a cracker, in particular a stream cracker, or a refining process. This C4 stream can comprise C4-alkanes in addition to the butenes. According to an embodiment of the invention, use is made of a C4 stream which consists of raffinate I. Raffinate I is here a fraction comprising 1-butene, cis/trans-2-butene, isobutene and also n-butane and iso-butane. For example, raffinate I can comprise 60-90% by weight of olefins and 10-40% by weight of butanes, with, for example, 10-40% by weight of 1-butene, 10-40% by weight of 2-butene and 10-50% by weight of isobutene. According to an embodiment of the invention, the C4 stream used has a butene content of from 20 to 100% by weight, preferably from 50 to 90% by weight, in particular from 70 to 90% by weight. The ratio of 1-butene to 2-butene is from 10:1 to 1:10, preferably from 3:1 to 1:3, in particular 2:1 1:2. The molar ratio of 1-butene+2-butene to isobutene is preferably from 10:1 to 1:5, in particular from 3:1 to 1:2. According to one embodiment of the invention, the C4 stream can contain small amounts of other hydrocarbons.
Raffinate I is preferably obtained by removal of butadiene from the crude C4 fraction obtained in steam cracking. This is achieved either by butadiene extraction with polar aprotic solvents (e.g. N-methylpyrrolidone) or by selective hydrogenation. Depending on the work-up, different C4 component distributions are obtained, for example (Figures in % by weight):
In principle, both types of raffinate I are suitable for the process of the present invention.
According to an embodiment of the invention, the starting material used can be any stream in which 1-butene, 2-butene and isobutene are present. According to one embodiment of the invention, the 1-butene and isobutene can originate from the synthesis of the present invention itself and be mixed with introduced 2-butene.
The C4 feed stream used is preferably pre-purified before use in the process of the present invention in order to remove any traces of water, oxygen or oxygenates, sulfur or sulfur-containing compounds, nitrogen, phosphorus or halogen, especially chlorides, which may be present. The removal is preferably carried out by passing the C4 feed stream over adsorber material such as zeolites and zeolite-like phosphates, high-surface-area oxides of silicon, aluminum, titanium, zirconium, bleaching earths, clays, hydrotalcites, high-surface-area phosphates, activated carbons and carbon molecular sieves and also organic polymers and ion-exchange resins, preferably NaX molecular sieve. The adsorber materials are preferably present as a guard bed.
Methods which can be used for adsorption and adsorptive purification are described, for example, in W. Kast, Adsorption aus der Gasphase, VCH, Weinheim (1988). The use of zeolitic adsorbents is explained in D. W. Breck, Zeolite Molecular Sieves, Wiley, N.Y. (1974). The removal of, specifically, acetaldehyde from C3-C15-hydrocarbons in the liquid phase is described in EP-A-0 582 901. The methods described in the above references can be used here. Thus, the feed stream in gaseous, liquid or supercritical phase is preferably brought into contact with the adsorbents.
Apart from the reaction of 1-butene, 2-butene and isobutene to form propene and 2-pentene and also 2-methyl-2-butene, a small proportion of 3-hexene, ethene, 2,3-dimethyl-2-butene and 2-methyl-2-pentene can be obtained as by-product. In addition, small amounts of higher-boiling compounds can also be present: 
In the above reaction, the desired molecular propene is formed in a cross-metathesis reaction of the xc3xa1-olefins 1-butene and i-butene with 2-butene. Approximately equimolar amounts of the C5-olefins 2-pentene and 2-methyl-2-butene are obtained as coproducts. Signficantly less favored are the respective self-metathesis reactions of 1-butene and isobutene (to form ethene and 3-hexene or 2,3-dimethyl-2-butene) and the cross-metathesis of the two xc3xa1-olefins (to form 2-methyl-2-pentene). The formation of the isobutene self-metathesis product 2,3-dimethyl-2-butene was not observed.
The C4+-olefins obtained in this first step can, after removal of ethene and propene by distillation, function as co-cracking feed with a significantly higher cracking value than the starting material. As an alternative, after separation of C4 and C5+, the propene yield can be increased by the ethenolysis of C5+ described below. Secondary components obtained in this reaction are preferably exclusively 1-butene and i-butene which can be recirculated to the first metathesis step or alternatively be isolated as pure materials using known technology. 
The small amounts of by-products in the first step, which according to an embodiment of the invention make up from 1 to 20% by weight, preferably from 1 to 5% by weight, of the amount of 2-pentene and 2-methyl-2-butene formed, do not interfere in the subsequent reaction (ethenolysis) so that, according to one embodiment of the invention, no purification of the 2-pentene and 2-methyl-2-butene to remove these by-products is necessary before the further reaction. According to one embodiment of the invention, the 2-pentene and 2-methyl-2-butene are used in pure form in the secondary reaction.
The expression xe2x80x9c2-pentene and 2-methyl-2-butenexe2x80x9d also includes those mixtures comprising not only 2-pentene and 2-methyl-2-butene but also small amounts of hexenes, in particular 3-hexene, and other higher-boiling compounds.
Correspondingly, the expression xe2x80x9cbutenesxe2x80x9d, like xe2x80x9c1-butenexe2x80x9d, xe2x80x9c2-butenexe2x80x9d and xe2x80x9cisobutenexe2x80x9d, also includes a mixture which comprises not only the butene or butenes but also C4-alkanes, in particular butanes.
A number of embodiments of the invention are illustrated below with the aid of the drawing, in which